Solid-state lighting from semiconductor LED light sources has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED-based lighting for their illumination applications. In this trend, more energy saving with a dimming control, more efficient CCT tunability, more environmental protection, and more aesthetic perception in lighting quality have become especially important and need to be well addressed.
The relationship between actual dimming and perceived dimming is not linear but logarithmic by nature because the human eye responds to low light levels by enlarging the pupil, allowing more light to enter the eye. This response results in a difference between measured and perceived light levels. For example, a lamp that is dimmed to 10% of its maximum measured light output is perceived as being dimmed to only 32%. Similarly, a lamp dimmed to 25% is perceived to be at 50%. Taking advantage of such differences, the use of a dimmer on LED-based lamps can save even more energy than actual dimming itself. Besides, reduced electrical consumption can further prolong life expectancy of the LED-based lamps and reduce maintenance or replacement costs.
A conventional wall-mount dimmer uses a leading-edge phase angle, trailing-edge phase angle, or phase cut to control a power delivering to a lighting device. Whereas such a dimmer seems to provide energy efficiency and is driving consumers to replace standard incandescent lamps with LED-based retrofit lamps, consumers often find that the performance they expect is not being achieved, at least when the solid-state lighting (SSL) products are used with existing TRIAC or phase-cut dimmers. Dimmer compatibility with LED-based lighting devices is a main issue. Basically, the wall-mount TRIAC dimmers are not so designed for LED loads that the existing residential wiring infrastructure can limit their capabilities for modern lighting controls. Furthermore, there are no industry standards that specifically guide LED dimming performance, and as such, a number of undesirable results may occur when one uses a dimmable LED-based lamp with an incandescent dimmer, such as reduced dimming range, flickering or strobing of the lamp, and inconsistent performance based on the number and classification of lamps being controlled by one incandescent dimmer. Moreover, a recent IEEE report raised a health concern due to invisible flicker at frequencies below 165 Hz including seizures, headaches, migraines, impaired ocular motor control, and impaired visual performance, etc.
Most of the existing residential and commercial electrical dimming infrastructures are single channel wall dimmers, which are crucial to serve the market with high quality solutions and to solve the various challenges to come. Furthermore, power factor of an electrical appliance refers simply to the degree to which the voltage potential and electric current draw required by the electrical appliance are in-phase for each half-cycle of the sinusoidal AC waveform. In fact, the current waveform should be in phase with AC voltage waveform to have a maximum power delivered to the load resulting in a unity power factor as in a purely resistive circuit. Conventional dimmers themselves have a major effect on power factor for all kinds of loads—capacitive, inductive, non-linear, and even linear and resistive, because such dimmers typically cut voltage phase over the current peak as required by the load, causing imbalance and harmonic distortion on the AC line. Poor power factor is rarely noticed by residential end-users because their utility companies usually pay the price by spending money on hardware and additional power to correct for this imbalance throughout their distribution systems. However, commercial users may either pay additional surcharges for low power factor or improve it at their own cost. For example, if their loads are highly inductive, they may have to install capacitor switch banks to compensate for this power loss.
A conventional driver employed to drive an LED-based lamp basically uses a switch-mode power supply (SMPS) and is considered to be nonlinear with reactive loads, which requires power factor correction (PFC) to reduce non-sinusoidal current distortion and excess energy at harmonics of the line frequency of the voltage. The EU standard EN61000-3-2 regulates harmonic contents and basic PFC criteria for all such switch-mode power supplies. Passive PFC in drivers/power supplies adopted in LED-based lamps usually involve adding capacitors, resistors and steering diodes in a valley-fill circuit. However, the power factor improvement using such a passive PFC circuit is limited. Active PFC involves redistributing the current over the voltage half-cycle waveform. The key is how to improve load regulation without adversely affecting the power factor or to make the load look like a linear resistor. Today, a conventional LED driver employing active PFC typically uses an energy transfer element that includes a flyback transformer to store energy which then directly provides LED current to an LED load. Although simple and low-cost, such a single-stage driver configuration provides so limited functionalities that can barely meet market demands. For example, market needs an external LED driver which can flexibly control one to several LED-based lighting devices in a luminaire. When part of lighting devices are removed from the luminaire for maintenance or replacement, an overall rated current can flow into the remaining LED-based lighting devices, resulting in excessive driving current for LED-based light sources. Market also needs an LED driver which can provide two or three sets of electric current to two or three types of LED-based light sources in order to control CCT of an LED lighting device that comprises such two or three types of LED-based light sources. The conventional LED driver can only provide single channel current control and thus fails to meet these market requirements.
Used as an early fluorescent dimming system and still used today, 0-10 V dimming has been employed to become one of reliable LED dimming control protocols although it is one of the earliest and simplest electronic lighting control signaling systems. A 0-10 V dimmer does not cut AC voltage for introducing phases and thus keep the AC voltage waveform intact. However, to control a dimming level of a lighting device using such a 0-10 V dimmer, one needs to have two extra low-voltage wires separately connected to the lighting device to be dimmed in addition to the power lines from the AC mains. This is so called 4-wire low voltage 0-10 VDC dimming. The low voltage control wires are polarity sensitive, and so accuracy is critical in wiring. This increases the wiring difficulty and installation cost, especially for the existing residential and commercial infrastructures that have two or three power wires in a wall-mount electrical box.
In today's lighting applications, CCT tuning is important. Although consumers demand a CCT tunable lamp that can tune from warm-white at 2,700 K, via sun-white and natural-white at 4,100 K, to cool-white at 6,200±300 K in general lighting to help improve the atmosphere in their working, exhibiting, or living areas, there have been very few such lighting products in luminaire markets. Manufacturers can generally make an LED-based lighting device using two kinds of phosphor coated white LEDs, one cool white and the other warm white, to mix light emissions with different ratios to come up with desired CCTs. However, the approach needs a proper LED driver to provide two sets of electric current with a proper ratio to the cool white and the warm while LEDs such as to emit a light emission with desired CCTs. A conventional driver apparently cannot meet such requirements.